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Posts

20 posts
Sections
19 sections
Actual post spacing
7.89 ft on centre
Pickets
328 pickets at 5.5 in
Rails
57 rails (3 per section)
Rail stock needed
450 linear ft
Concrete
80 bags of 50 lb (4 per post)
Post length needed
8.5 ft minimum (6 ft above grade + 2.5 ft buried)
  • 19 sections at 7.89 ft, not 18 at 8 ft — because 18 sections would only span 144 ft of a 150 ft fence. The section count rounds up and the spacing comes out below your maximum.
  • 20 posts for 19 sections — one more than the gaps, because both ends need a post. This is the error that leaves you one post short on delivery day.
  • Pickets are calculated on the actual 5.5 in width, not the nominal size. Treating a 1 × 6 as 6 inches wide would give 300 pickets instead of 328 — short by 28.
  • Post holes must reach below your local frost line, which in cold regions is deeper than the one-third rule gives. A post set above frost depth will heave, and the whole fence follows it.

Posts by maximum spacing

A tighter spacing means more posts but a stiffer fence — worth it in exposed or windy locations.

Maximum spacingSectionsPostsActual spacing
5 ft30315 ft
6 ft25266 ft
7 ft22236.82 ft
8 ft (yours)19207.89 ft
9 ft17188.82 ft
10 ft151610 ft

How many fence posts do I need?

Divide the fence length by the maximum post spacing, round up for the section count, then add one for the posts. A 150 ft fence at 8 ft maximum spacing needs 19 sections at 7.9 ft and 20 posts, because both ends need a post.

sections = ⌈length ÷ max spacing⌉ posts = sections + 1 pickets = ⌈length × 12 ÷ (picket width + gap)⌉
⌈ ⌉
Round up — rounding down leaves part of the run unspanned
max spacing
The furthest posts may be apart; actual spacing comes out lower
picket width
Actual, not nominal — a 1 × 6 board is 5.5 inches

Three things here are routinely got wrong, and all three cost material. First, the section count rounds up, not down. A 150 foot fence at a maximum 8 foot spacing does not give 18 sections of 8 feet — that spans only 144 feet. It gives 19 sections at 7.9 feet each, and the real spacing always comes out at or below the maximum rather than exactly on it. Second, posts are a fencepost problem: there is always one more post than there are sections, because both ends of the run need one. Counting posts equal to sections is the classic off-by-one and it leaves you a post short. Third, pickets must be calculated on actual board width. A nominal 1 × 6 is 5.5 inches wide, so using 6 inches under-orders by 9% — on this fence that is 28 pickets missing. Corners and gates add posts beyond the formula: each corner needs its own post and each gate needs two, so measure each straight run separately and add them.

  1. Measure each straight run separately and add the lengths.
  2. Divide by the maximum spacing and round the result up.
  3. Add one to the section count for the number of posts.
  4. Multiply sections by rails per section for the rail count.
  5. Divide the length in inches by picket width plus gap.
  6. Calculate hole volume less the post to get concrete needed.

Worked example: A 150 ft privacy fence, 6 ft high, 8 ft maximum spacing, 1 × 6 pickets with no gap

A 150 ft privacy fence, 6 ft high, 8 ft maximum spacing, 1 × 6 pickets with no gap

Dividing 150 ft by the 8 ft maximum gives 18.75, which rounds up to 19 sections — not down to 18, because 18 sections of 8 ft would span only 144 ft and leave 6 ft of fence with nothing holding it. The actual spacing is therefore 150 ÷ 19 = 7.89 ft, comfortably inside the maximum. Posts come to 20, one more than the sections, because both ends of the run need one. At three rails per section that is 57 rails and 450 linear feet of rail stock. Pickets are where nominal sizing bites: a 1 × 6 board is actually 5.5 inches wide, so 150 ft × 12 ÷ 5.5 = 328 pickets. Using 6 inches would have given 300 — 28 short. Each 10 inch diameter hole 30 inches deep holds 1.364 cubic feet, less 0.213 for the post itself, so 1.151 cubic feet of concrete per post. At 0.375 cubic feet per 50 lb bag that is 4 bags per post and 80 bags in total.

What each fence input means

Total fence length
The total run. For a fence with corners, measure each straight run and add them — each corner needs its own post.
Maximum post spacing
The furthest apart posts may be. Eight feet is standard for wood; six gives a stiffer fence in windy or exposed locations. Actual spacing comes out at or below this.
Fence height
Above-grade height. The post must be about a third as long again to set below frost depth, so a 6 ft fence needs an 8 or 9 ft post.
Rails per section
Two for a fence up to about 4 ft, three for 5 to 6 ft, four above that. Rails carry the pickets and stop them cupping.
Picket type
Use the actual width, not the nominal. A 1 × 6 board is 5.5 inches wide — calculating it as 6 inches under-orders by 9%.
Gap between pickets
Zero for a solid privacy fence. A spaced picket fence commonly uses a gap equal to the picket width.
Concrete bag size
Yields are 0.375, 0.45 and 0.6 cubic feet respectively.
Post hole diameter
Roughly three times the post width. A 4 × 4 post wants a 10 to 12 inch hole.
Post hole depth
A third of the above-grade height is the rule, but it must also reach below your local frost line — which may be the binding constraint.

What this fence does not account for

  • Round the section count up, never down. Eighteen sections at 8 ft span only 144 ft of a 150 ft fence, leaving part of the run unsupported.
  • There is always one more post than sections, because both ends need one. Counting them equal is the classic off-by-one error on a fence order.
  • Pickets must use actual board width. A nominal 1 × 6 is 5.5 inches, so calculating at 6 inches under-orders by 9%.
  • Each corner needs its own post and each gate needs two. Measure every straight run separately and add them, rather than treating a perimeter as one length.
  • Post holes must reach below the local frost line, which in cold regions is deeper than the one-third-of-height rule. A post above frost depth will heave and take the fence with it.
  • Gate posts carry a swinging load and should be larger — a 6 × 6 rather than a 4 × 4 — set deeper and in more concrete than line posts.
  • Sloping ground means either stepped panels or racked rails. Either way the rails and pickets need more length than the horizontal measurement suggests.
  • Check the property line and local setback rules before setting posts. Moving a concreted fence is considerably harder than measuring twice.

Frequently asked questions

How many posts for a 150 ft fence?

Twenty, at 8 ft maximum spacing. That is 19 sections at an actual 7.89 ft apart, plus one extra post because both ends of the run need one.

How far apart should fence posts be?

Eight feet is standard for a wood fence, and six gives a stiffer result in windy or exposed locations. Treat it as a maximum — the actual spacing comes out lower once the count rounds up.

How deep should fence posts be?

About a third of the above-grade height, so 24 to 30 inches for a 6 ft fence — but it must also reach below your local frost line, which may require considerably more in cold regions.

How many pickets do I need per foot?

With 1 × 6 boards at their actual 5.5 inch width and no gap, about 2.2 pickets per foot. A 150 ft fence needs 328. Using the nominal 6 inches would under-order by 9%.

How much concrete per fence post?

A 10 inch hole 30 inches deep needs about 1.15 cubic feet after deducting the post, which is 4 bags of 50 lb mix or 3 bags of 60 lb.

How many rails does a fence need?

Two for fences up to about 4 ft, three for 5 to 6 ft, and four above that. Rails carry the pickets and stop them cupping, so under-railing shows within a season.

Sources and review

Last reviewed .